Coating method and apparatus therefor



June 1953 E. E. CHADSEY, JR,, ETAL 2,643,201

COATING METHOD AND APPARATUS THEREFOR Filed Dec. 24, 1949 Wire Fuder FIG. I

- Skin dnpfh FIG.

INVENTORS far/ 5. Chadsey Jr. Phi/[p J C/ouqb Phi/[p Gad/e 2' ATTORNEY Patented June 23, 1953 COATING METHOD AND APPARATUS THEREFOR Earl E. Chadsey, Jr., Newton, Philip J. Clough, Reading, and Philip Godley 2nd, Lexington, Mass., assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Application December 24, 1949, Serial No. 134,988

2 Claims.

This invention relates to coating and more particularly to the coating of various substrate materials with aluminum by vacuum evaporation and deposition of aluminum. This application is a continuation-in-part of our copending application, Serial No. 117,124 filed September 22, 1949.

A principal object of the present invention is to provide improved processes for evaporating molten aluminum for the purpose of providing a coat of aluminum on a substrate positioned in a vacuum chamber.

Another object of the invention is to provide a process of the above type which permits high evaporation rates over long periods of operation of an aluminum-evaporating crucible.

Still another object of the invention is to provide a process of the above type which permits the use of a carbonaceous crucible for evaporating aluminum, the formation of aluminum carbide being rendered ineffective to create a scum which might interfere with rapid evaporation of the aluminum.

Still another object of the invention is to provide an apparatus which is particularly adapted for performing processes of the above type.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts, and the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

Fig. 1 is an exaggerated, diagrammatic, partially sectional view of one preferred form of the invention; and

Fig. 2 is an exaggerated, enlarged, sectional view of a portion of Fig. 1.

In the coating of materials such as paper, synthetic plastics and the like with an aluminum film by vacuum evaporation of the aluminum, it is desirable to use a relatively small crucible so that radiant heat from the area of the molten aluminum does not unduly raise the temperature of the substrate to be coated. It is also desirable to feed aluminum into the crucible during the coating operation so as to provide for long periods of operation of the coating equipment. It is additionally advantageous to employ crucible materials which are relatively inexpensive and which have a relatively long life. Additionally, it is extremely desirable to have crucible materials which are not readily subject to heat shock so as not to unduly lengthen the warm-up and cool-off periods of the crucible.

It has been found that refractory carbonaceous materials, such as elemental carbon, for example amorphous carbon, graphite and, particularly, mixtures of amorphous carbon and graphite, have certain excellent characteristics such as cheapness, ease of machinability, and resistance to heat shock when used as crucibles for coating aluminum. However, molten aluminum, when raised to temperatures on the order of 1209 to 1306 C., rapidly attacks such crucibles with the formation of aluminum carbide. This aluminum carbide is soluble in molten aluminum, but the concen tration thereof quickly approaches the saturation point. As long as the aluminum carbide remains in solution, it does not seriously interfere with the coating operation. However, the surface of the molten aluminum from which the evaporation takes place is cooler than the main body of the molten aluminum. Therefore, at this surface, the aluminum carbide tends to come out of solution with the formation of small crystals of aluminum carbide. These crystals, being lighter than aluminum, float on the surface of the molten aluminum in the form of a scum. This scum decreases the effective area from which the alu-- minum can be evaporated, thereby cutting down the evaporation rate. This defect is particularly noticeable in those cases where solid aluminum is fed to the molten bath during the course of evaporation. Since the solid aluminum is at a temperature considerably below the temperature of the bath, chilling of the surface occurs when the solid aluminum strikes the bath. This chilling makes the scum formation even more readily" apparent. The cool metal, being heavier than the hot metal, sinks rapidly to the bottom of the crucible where it reacts with more carbon to form additional aluminum carbide. Thus the feeding of aluminum to the molten pool of aluminum first causes the precipitation of aluminum carbide in the form of a scum and, additionally, forms more aluminum carbide which is dissolved in the melt. The melt continuously remains saturated with aluminum carbide and more and more scum is formed as the solid aluminum is fed to the melt. After a relatively short period the scum formation may be so severe as to completely stop evaporation of aluminum.

It has been found that the above-mentioned disadvantages may be overcome, and that greatly increasedevaporation rates and greatly increased evaporating times may be achieved by providing for a stirring of the molten aluminum during the evaporation thereof. This stirring has the effect of maintaining the temperature more nearly uniform throughout the body of the molten aluminum and, additionally, has the effect of breaking up any scum which commences to form on the surface of the aluminum, carrying the formed crystals of aluminum carbide down tothe hotter portions of the melt Where they are again 'parv tially dissolved. The stirring thus acts to keep the aluminum melt saturated with aluminum carbide previously formed, rather than by the formation of new aluminum carbide resulting from the addition of fresh aluminum metal.

In a preferred form'of the invention .the stirring is achieved by inducing eddy currents in the molten aluminum melt, these eddy cur-rents'being preferablyproduced by an induction coil which serves also to heat the crucible and the melt to the desired high temperature necessary forrapid evaporation of the aluminum. Since the carbonaceous materials employed for the crucible act as an electromagnetic shield for the melt, it is necessary to provide, in a preferred form of the invention, a thickness of crucible wall that is less than the skin depth of the induced current. In this way part of the induced current flows within the aluminum itself. As a result of the induced currentflowing in the molten aluminum, liquid currents are. generated which maintain stirring. of the molten aluminum.

Skin depth of aninduced current is a function of both the frequency of the currentbeing used and the resistivity of the crucible material. Skin depth may be calculated by the method shown on page 34 of Radio Engineers Handbook by Terman, First Edition, McGraw-Hill, 1943. The equation given by Terman is. asfollows:

skin depth in cm. =5,033 /fi where p is resistivity in ohms per cm. cube, 7 is cycles per second and a is magnetic permeability. Another standardtext (Stratton iElectromagnetic Theory First. Ed. vp. .403, McGrawr-Hill) gives a similar method of calculating. skin depth. The following table gives .the skin depth ofinduced currents with carbon and. graphite at 1300 C. at several different frequencies.

TABLE I Skin depth in inches (1300 C.)

Frequency Carbon Graphite 9,600 cycles/sec .88 63 100,000cycles/scc .27 .20 150,000 cycles/sec. ..23 ..l7

an. electromagnetic field which. causes physicalv 4 flow of the molten aluminum, this flow being generally axially of the crucible, the metal tending to concentrate in the center of the crucible.

In a preferred form of the invention the agitation of the molten aluminum. is not sufficient to cause splashing of the aluminum out of the crucible. However, the stirring is sufficiently vigorous so as to provide for thorough remixing of aluminum carbide crystals formed on the surface of the melt so that these crystals do not collect as a scum on the melt surface.

Referring now to Figs. 1 and 2, there is shown one. preferred embodiment of the invention wherein 10 represents a vacuum-tight housing providing a vacuum chamber l2, this chamber being kept at a pressure in the micron range by means of a vacuum pumping system schematically indicated at [4. Within the chamber [2 there is provided a means for supporting a base material to be coated, this means being shown schematically as a first spool i6 and a second spool 18 carrying therebetween the base material 20.

The means for vaporizing the metal comprises a metal-holding, carbonaceous crucible 22 having a main body portion 24, in which the aluminum 26 is to be held in molten condition and heated to a temperature sufiicient to vaporize the aluminumat a high rate under the pressure existing in the vacuum chamber 12. Extending from the top of the main body portion 24, there is provided a lip 28 which is preferably formed integrally with the body portion 24. For providing heat to the aluminum 26 and the metal-holding crucible 22, there is included a heating means, schematically indicated at 32. This heating means preferably comprises an induction coil having a portion 34 for heating the main body of the crucible, and the aluminum carried thereby, and a.portion 36 for heating the lip 28 of the metal-holding crucible. A suitable power supply 38 is provided for furnishing a high frequency current to the induction coil 34, 36. As can be seen from Fig. 1, the turns in portion 36 of the coil, adjacent the lip 28, are closer together than are the turns in portion 34 of the coil, adjacent the main body portion of the crucible.

For preventing radiation heat loss from the crucible 22, there is provided an outer crucible 40, preferably made of a refractory material which does not conduct electricity, and a refractory packing 42 between the metal-holding crucible 22 and the outer refractory crucible 40. Means for feeding metal to the crucible are also preferably provided. Such means may comprise a wire-feeding mechanism schematically indicated at 4| which feeds wire to the crucible at a rate equal to the rate of evaporation of the aluminum.

In a preferred form of the invention, the metal-holding crucible 22 is preferably formed of a current-conducting, carbonaceous material, such as carbon, graphite or a graphite-carbon mixture of the type commercially available under the trade name Graphitar. The outer crucible 40 preferably comprises a refractory material, such as silica, while the refractory packing 42 preferably comprises zirconia or beryllia. The induction coil, in a preferred form, comprises a refractory tubing 4 5, such as Pyrex, quartz, or the like, Within which is 1 sitioned a multistranded copper conductor 46, there being sufficient-space between the copper conductor 46 and the wall of the. tubing 44t:to permit the circulation of cooling water 43 therebetween. Thisarrangem'ent has the particular advantage.v of preventing arc discharge between the turns of the coil at the relatively high vacuum employed. The induction coil is preferably provided with a suitable source of high frequency current of approximately 9,000 to 150,000 cycles per second from a usual high frequency power source. When a Graphitar crucible 22 is employed for vaporizing aluminum, and a frequency of about 100,000 cycles is used, the skin depth of the induced current is approximately 0.2 inch at the temperature employed. The wall of the crucible is thus preferably made slightly thinner than 0.2 inch (for example .125 inch) so that some of the induced current flows in the molten aluminum charge.

As a result of the induced eddy currents in the melt, the aluminum is subjected to an alternating electromagnetic field which tends to concentrate the aluminum in the center of the crucible. The action of gravity prevents complete concentration so that the top central portion of the melt rises up into the cone 2%, with the creation of circulatory flow of aluminum in the general direction shown by the arrows 21. This flow of aluminum causes a thorough mixing of the melt which prevents the formation of a scum of aluminum carbide crystale and also acts to carry the formed crystals of aluminum carbide from the surface of the melt down into the body thereof where at least some redissolving of the aluminum carbide takes place.

As can be seen best in Fig. 2, a portion of the molten aluminum tends to creep up the side of the crucible 24 and onto the lip 28, this creeping meniscus of aluminum being indicated at 26a. In the preferred form of arrangement of crucible lip and heating coil 36, the lip 28 is maintained substantially hotter than the body portion of the crucible 24 so that aluminum in the creeping meniscus 26a is evaporated prior to the time when it can creep across the surface of the lip 28. This feature of the invention is described more fully and claimed in the above-mentioned application, Serial No. 117,124, filed September 22, 1949.

Since certain changes may be made in the above apparatus and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description; or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a process for the coating of a substrate with aluminum by the vapor deposition of aluminum in a vacuum chamber wherein the aluminum is evaporated from a carbonaceous crucible which is formed of elemental carbon selected from the group consisting of amorphous carbon, graphite and mixtures thereof, the improvement which comprises heating the crucible and the molten aluminum therein to a temperature above about 1200 C. so that the vapor pressure of the aluminum is above the free air pressure in the vacuum chamber while supplying induced heating current to the crucible and the aluminum therein, maintaining said induced heating current at a frequency between about 9600 and 150,000 cycles per second, said frequency and the crucible wall thickness being so related that the skin depth of the induced current is substantially greater than the thickness of the crucible wall to cause a substantial stirring of said molten aluminum, thereby carrying into the body of the melt aluminum carbide crystals floating on the surface of the melt and thus maintaining the surface of the aluminum melt free of substantially all aluminum carbide scum, and depositing on said substrate aluminum vapors emanating from the surface of the molten aluminum.

2. Apparatus for coating a base material with aluminum by vapor deposition techniques, said apparatus comprising a vacuum-tight chamber, means for positioning said base material within said chamber, means for evacuating said chamber to a low free air pressure, a carbonaceous crucible for melting and vaporizing said aluminum, said carbonaceous crucible being formed of elemental carbon selected from the group consisting of amorphous carbon, graphite and mixtures of amorphous carbon and graphite, an induction coil for heating said crucible and the aluminum held thereby to a temperature above about 1200 C. so that the vapor pressure of the aluminum is above the free air pressure in said chamber, means for feeding aluminum to said crucible, means for advancing said substrate past the source of aluminum vapors to coat said substrate with aluminum, and a source of alternating current energy for said induction coil, the frequency of said current being between about 9600 and 150,000 cycles per second, the thickness of the crucible being about to the skin depth of the electric current induced in the crucible and the molten aluminum, whereby a substantial proportion of the induced current flows in the molten aluminum to cause a thorough stirring of the molten aluminum with remixing into the body of the molten aluminum of aluminum carbide scum floating on the surface of the molten aluminum.

EARL E. CHADSEY, JR. PHILIP J. CLOUGH. PHILIP GODLEY 2ND.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,601,523 George Sept. 28, 1926 1,694,792 Northrup Dec. 11, 1928 1,872,990 Linnhoif Aug. 23, 1932 2,255,549 Kruh Sept. 9, 1941 2,378,476 Guellich June 19, 1945 2,384,500 Stoll Sept. 11, 1945 2,467,058 Tama Apr. 12, 1949 2,513,082 Dreyfus July 27, 1950 2,584,660 Bancroft Feb. 5, 1952 FOREIGN PATENTS Number Name Date 590,537 France Mar. 19, 1925 

1. IN A PROCESS FOR THE COATING OF A SUBSTRATE WITH ALUMINUM BY THE VAPOR DEPOSITION OF ALUMINUM IN A VACUUM CHAMBER WHEREIN THE ALUMINUM IS EVAPORATED FROM A CARBONACEOUS CRUCIBLE WHICH IS FORMED OF ELEMENTAL CARBON SELECTED FROM THE GROUP CONSISTING OF AMORPHOUS CARBON, GRAPHITE AND MIXTURES THEREOF, THE IMPROVEMENT WHICH COMPRISES HEATING THE CRUCIBLE AND THE MOLTEN ALUMINUM THEREIN TO A TEMPERATURE ABOVE ABOUT 1200* C. SO THAT THE VAPOR PRESSURE OF THE ALUMINUM IS ABOVE THE FREE AIR PRESSURE IN THE VACUUM CHAMBER WHILE SUPPLYING INDUCED HEATING CURRENT TO THE CRUCIBLE AND THE ALUMINUM THEREIN MAINTAINING SAID INDUCED HEATING CURRENT AT A FREQUENCY BE TWEEN ABOUT 9600 AND 150,000 CYCLES PER SECOND, SAID FREQUENCY AND THE CRUCIBLE WALL THICKNESS BEING SO RELATED THAT THE SKIN DEPTH OF THE INDUCED CURRENT IS SUBSTANTIALLY GREATER THAN THE 